document why the ppx can't be the same for melange and native

at first glance it's not obvious why a melange project can't just use the usual ppx_deriving_yojson/ppx_yojson_conv. And then why melange-json exposes 2 different ppx depending on the compilation target.

Don't have time now to write proper docs, but the main reason is that in the browser you can leverage Json.parse, which can be faster than any other alternative, and requires much less runtime.